EMMagnetism 1 Magnetism Lodestone : naturally occurring mineral ore For certain kind of substance, they attract irons and other special kinds of metal. Moreover, they have tendency to align themselves along N-S direction. Next Slide North pole : the end always pointing north South pole : the end always pointing south Like poles repel each other; unlike poles attract each other Photo

EMMagnetism 2 Magnetic fields Force experienced by iron exists near the magnet and is strongest near the poles. This is magnetic field. Magnetic field lines : A compass aligns itself along the field lines near the magnet. Next Slide Common magnetic field patterns Effect of magnetic field on iron powder and small compass Diagram Photo

EMMagnetism 3 Magnetic effect of current Magnetic field pattern due to a straight wire with current Right-hand grip rule for direction of field Next Slide Solenoid and coil : circular wire with a number of turns Magnetic field pattern due to a solenoid with current Right-hand grip rule for solenoid Diagram Photo Diagram

EMMagnetism 4 Electromagnets Diagram for an electromagnet Next Slide Electromagnetic cranes D.C. electric bells Ticker-tape timers Telephone communication Diagram Photo

EMLeft hand rule 1 Fleming’s left hand rule When a wire carrying a current is placed in a magnetic field, it experiences a force. Fleming’s left hand rule Next Slide Loudspeaker Turning effect on a coil in magnetic field Simple d.c. motor Diagram

EMLeft hand rule 2 Applications Practical motors Moving-coil galvanometer Next Slide Change a galvanometer into an ammeter by a shunt resistor Change a galvanometer into a voltmeter by a multiplier resistor Multimeter Photo Diagram Photo

END of EM

EMMagnetism 1 Some sample magnets are shown in the following photo. Back to Click Back to

EM Next Slide Magnetism 2 Magnetic field pattern of a bar magnet The direction of field lines can be represented by a small north pole object, like the north pole of a small compass. NS small compass

EMMagnetism 2 Magnetic field pattern between two large poles Click Back to NS Back to

EMMagnetism 2 Effect on magnetic powder Next Slide

EMMagnetism 2 Effect on small compass Click Back to Back to

EMMagnetism 3 Magnetic field pattern due to a straight wire Next Slide current flowing into the paper current flowing out of the paper

EM Click Back to Magnetism 3 Effect of current in a straight wire on small compass Back to Without current With current

EM Click Back to Magnetism 3 Right-hand grip rule for direction of field The right hand’s fingers grip in the direction of the field if the the thumb points to the same direction as the flow of current. Back to Direction of current Direction of magnetic field

EM Click Back to Magnetism 3 Solenoid and coil Back to

EM Click Back to Magnetism 3 Magnetic field of a solenoid with current Back to current field lines

EM Next Slide Magnetism 3 Right-hand grip rule for solenoid If the thumb of the right-hand points to the N-pole, the fingers would point to the direction of current flow in the coil. N-poleS-pole fingers indicate current direction

EM Click Back to Magnetism 3 Strength of magnetic field of a solenoid can be increased by.N-poleS-pole fingers indicate current direction (i) increasing the current, and (ii) increasing the number of turns per unit length Back to

EM Click Back to Magnetism 4 An electromagnet with soft iron-core’s field pattern : Back to current field lines soft iron

EM Click Back to Magnetism 4 D.C. electric bells Back to

EM Click Back to Magnetism 4 Ticker-tape timers Back to

EM Click Back to Magnetism 4 Telephone communication Back to

EM Next Slide Left hand rule 1 Fleming’s left hand rule : The thumb, the first finger and the second finger indicates the direction of force (F), magnetic field (B) and current (I) respectively if they are held perpendicular to each other. (FBI in short) Current (I) Force (F) Magnetic field (B)

EM Click Back to Left hand rule 1 Force experienced by the current-carrying conductor can be increased by Back to (a) increasing the strength of the magnetic field, (b) increasing the size of the current, (c) increasing the length of the conductor. NS Force Current is flowing out of the paper

EM Next Slide Left hand rule 1 Loudspeaker paper cone magnet solenoid electrical signal N NN N S

EM Click Back to Left hand rule 1 Varying alternating current passes through the coil. Back to paper cone vibrates to produce sound magnet Solenoid with paper cone is forced to vibrate due to the force in left hand rule electrical signal

EM Next Slide Left hand rule 1 A coil with current flow is placed in a uniform field as shown in the following figure. NS

EM Next Slide Left hand rule 1 When the plane of the coil is parallel to the field, the couple produced is greatest and the coil rotates clockwisely. NS NS As the coil rotates, the perpendicular distance between the two forces becomes smaller and so the couple decreases.

EM Next Slide Left hand rule 1 NS NS When the plane of the coil is perpendicular to the field, there is no couple. It still rotates clockwisely due to its inertia. When the coil overshoot the vertical, anti-clockwise moment turns the coil back.

EM Click Back to Left hand rule 1 Back to The coil, at last, oscillates about the vertical line. NS

EM Next Slide Left hand rule 1 A simple d.c. motor contains a rectangular coil of many turns which can freely rotate about an axis. Uniform magnetic field which is produced by using two large magnets, passes through the coil. The ends of this coil contains two half-rings which are in contact with two small carbon brushes. The half-rings are called as commutators.

EM Next Slide Left hand rule 1 Couple produced forces the coil to rotate clockwisely

EM Next Slide Left hand rule 1 As the coil rotates, the turning couple decreases.

EM Next Slide Left hand rule 1 When the coil passes the vertical line, the carbon brushes are in contact with the rings of the opposite sides. The direction of the current as well as the direction of the forces reverses. The coil therefore carries on rotating clockwisely.

EM Click Back to Left hand rule 1 Turning effect on the coil and the simple d.c. motor can be increased by Back to (a) increasing the flow of current (b) increasing the number of turns in the coil (c) increasing the strength of the magnetic field (d) increasing the area of the coil

EM Click Back to Left hand rule 2 Practical motor Back to

EM Next Slide Left hand rule 2 Moving-coil galvanometer

EM Click Back to Left hand rule 2 Sensitivity of a galvanometer can be increased by Back to (a) using weaker hairsprings (b) increasing the number of turns in the coil (c) increasing the strength of the magnetic field (d) increasing the area of the coil Full scale deflection current (f.s.d. current) is the current needed to deflect the pointer to the end of the scale.

EM Next Slide Left hand rule 2 An ammeter can be made by connecting a resistor in parallel with a galvanometer. This ammeter can measure larger current. The resistor used in this case is called shunt resistance. shunt resistance galvanometer

EM Next Slide Left hand rule 2 A milliammeter has a resistance of 5  and a f.s.d. current of 10 mA. What is the value of the shunt resistance needed to convert the meter to measure currents up to 1 A? What is the resistance of the adapted meter? shunt resistance (S) Resistance = 5  1 A 10 mA A = 0.99 A MN

EM Next Slide Left hand rule 2 We assume 1 A current passes the ammeter. Back to Current through the milliammeter = f.s.d. current = 0.01 A Current through the shunt = ( ) A = 0.99 A p.d. across MN = 0.01  5 = 0.99  S  S =   Resistance of the adapted ammeter = 0.1  (Why?)

EM Next Slide Left hand rule 2 A voltmeter can be made by connecting a resistor in series with the galvanometer. The resistor used in this case is called multiplier resistance. multiplier resistance galvanometer A B

EM Next Slide Left hand rule 2 A milliammeter has a resistance of 5  and a f.s.d. current of 20 mA. What is the value of the mulitplier resistance needed to convert the meter to measure p.d. up to 10 V? What is the resistance of the adapted meter? Multiplier (R) A B 20 mA 5  p.d. = 10 V

EM Next Slide Left hand rule 2 We assume 10 V p.d. is across the voltmeter. Back to Current through the multiplier and the galvanometer = f.s.d. current = 0.02 A p.d. across AB = 10 V = 0.02  (R + 5)  R = 495   Resistance of the adapted meter = 495  + 5  = 500 

EM Next Slide Left hand rule 2 Multimeter Back to